Technical Field
The present disclosure relates to semiconductor devices, especially a graphene-based valley filter which is capable of switching valley polarizations through electrical gate control.
Description of Related Art
Graphene has many extraordinary properties and has been considered as a promising future material. Graphene has mechanical strength about 200 times stronger than steel, and conducts heat and electricity with great efficiency. Furthermore, it is also identified to have advantageous electrical properties such as bipolar transistor effect, ballistic transport of charges and large quantum oscillations. Carrier mobility of Graphene sheets may be greater than 200,000 cm2/V-sec at 4K and as high as 15,000 cm2/V-sec even in 300K. Thus it opens a great potential on manufacturing high speed electronic devices.
Structurally, graphene is an allotrope of carbon in the form of a two-dimensional, one atomic layer thin, hexagonal lattice in which every atom occupies a vertex. Graphene has hybrid orbitals formed by sp2hybridization. The 2s orbital and two of the three 2p orbitals are mixed to form three sp2orbitals. The one remaining p-orbital forms a pi (π)-bond between the carbon atoms. Fundamentally, the electronic properties of graphene are dictated by the bonding and anti-bonding orbitals (giving respectively the valance and conduction bands) derived from these p-orbitals.
Some unusual properties of graphene may lead to difficulties in manufacturing desired electronic devices. For example, a graphene sheet has a zero bandgap with linear energy-momentum relation for carriers in its nature state. This makes it impossible to produce electric charge switching effect in graphene as a conventional transistor. Therefore, opening bandgap in graphene remains a main challenge before making it into a switching semiconductor device such as FET (Field-Effect Transistor).
Moreover, it has been established recently that, in addition to charge and spin, electrons in graphene also have a valley pseudospin degree of freedom that can be exploited to create valleytronic devices. This property is derived from the existence of multiple minima (maxima) of conduction (valence) bands in momentum space. Generally speaking, promising systems to explore the valley-related properties are two-dimensional honeycomb lattices including gapped graphene systems and monolayer transition metal dichalcogenides.
Probing and controlling the valley degree of freedom in graphene systems by transport measurements has been a major challenge to fully exploit the unique properties of this two-dimensional material.
Central to valleytronic applications are the devices that generate valley-polarized electrons for valleytronic signal processing. The first experimental demonstration of valley polarization was achieved by optical pumping with circularly polarized light in monolayer MoS2 by using the valley-contrasting selection rules for optical transitions in the K and K′ valleys. On the other hand, possible electrical valley filters in graphene systems have been proposed with filtering mechanisms based on, for example, the valley dependence of quantized states in a zigzag graphene nanoribbon, the valley-dependent electron scattering at a line defect, breaking the valley symmetry by employing strain and magnetic field simultaneously or an AC external field, and utilizing the energy band warping. Experimental implementation is not yet reported for various reasons: some of the proposed configurations could not be easily constructed in the laboratory, may not have switchable polarity, or require the presence of magnetic fields. Hence it leaves plenty of room for further improvement in order to make valleytronics a foreseeable reality.
From the application point of view, it is highly desirable to integrate the valleytronic components with conventional electronic components (e.g., integrated electrovalleytronics). This approach allows for the application to take full advantage of each component's unique performance, for example, using electronic devices for storage units to circumvent the valley decoherence problem in valleytronic devices, and using valleytronic devices for processing units to mitigate the power consumption problem in downscaling integrated circuits.